With your fiddling with regex, you can make whole sections of text disappear in such a way that you won't see them until you have invested some amount of time afterwards in further editing.

With this kind of experimentation, you want to save with a new name every five minutes at least.

If there is someone else who is familiar with your material, it would be beneficial for them to double check you because after fiddling for an hour or so, you will not see such changes or maybe even care!